The dip coating method has been used to manufacture certain types of photoreceptors. In this dip coating method, a cylindrical substrate is immersed in a vessel containing the solution to be coated; the substrate is then withdrawn at a rate that controls the coating thickness. The vessel also may be cylindrically shaped. The usual procedure used to fill the vessel is to pump a coating solution containing the active materials, either dissolved or in suspension such as pigments, into the vessel from the bottom and continuously overflow the vessel at the top. In this way, the substrate is subjected during dip coating to a uniform flow of coating solution relative to the coating speed.
Conventional dip coating methods and apparatus are deficient in at least one respect. FIG. 1 depicts a conventional dip coating apparatus 2 where the movement of the coating solution is indicated by flow lines 4. As seen in FIG. 1, the coating solution flows from the bottom of the vessel 6 into an overflow container 8 positioned adjacent the vessel. However, a zone 10 of stagnant coating solution is present as indicated by the flow lines around the zone 10 and the absence of flow lines within the zone. The term xe2x80x9cstagnantxe2x80x9d means no movement of coating solution or relatively little movement. Contaminants (e.g., dirt, chips, and agglomerations) may be present in the zone. Because of the relative surface tension between the substrate 12 and the coating solution, such contaminants near the point of contact of the substrate and the coating solution can be pulled onto the substrate surface during dip coating. There is then a high probability that the contaminants will remain on the substrate after dip coating which can degrade the quality of the dip coated layer.
Thus, there is a need, addressed by the present invention, for new dip coating methods and apparatus which avoid or minimize the above discussed deficiency.
Conventional dip coating methods and apparatus are described in Swain et al., U.S. Pat. No. 6,132,810 and Petropoulos et al., U.S. Pat. No. 5,725,667.
The present invention is accomplished in embodiments by providing a method comprising:
(a) overflowing a vessel with a coating solution, wherein there is a zone of stagnant coating solution within the vessel;
(b) directing at least one fluid jet at the zone of the stagnant coating solution, thereby causing at least a portion of the stagnant coating solution to overflow the vessel; and
(c) dip coating a substrate with the coating solution in the vessel, subsequent to the feature (b).